Process for preparing aluminum base alloys



United States Patent 3,418,177 PROCESS FOR PREPARIYG ALUMINUM BASE ALLOYS Michael J. Pryor, Hamden, Conn, assignor to Olin Mathieson Chemical Corporation, a corporation of Virginia N0 Drawing. Filed Oct. 14, 1965, Ser. No. 496,147

12 Claims. (Cl. 148-115) ABSTRACT OF THE DISCLOSURE An improved process for preparing aluminum base alloys in wrought form, especially conductors, wherein the alloy contains magnesium and silicon including the steps of holding at an elevated temperature, hot rolling with a cooling rate during hot rolling of greater than 100 F. per minute and cooling to below 250 F. at a rate greater than 100 F. per minute with less than 20 seconds delay between said cooling and said hot rolling.

The present invention relates to an improved process for the manufacture of aluminum base alloys. More specifically, the present invention resides in a greatly improvide process for the preparation of aluminum base alloy conductors wherein the alloy contains magnesium and silicon.

The present invention relates specifically to heat treatable aluminum alloys in which the major portion of alloying elements are magnesium and silicon which will be principally in the form of the intermetallic compound magnesium silicate, Mg si.

Alloys of this type are widely used for a variety of applications, including use in alluminum alloy conductors. For example, aluminum alloy 6201 is a high strength aluminum-magnesium-silicon alloy which is the heat treated condition shows a tensile strength of about 46,000 p.s.i. minimum, and elongation of 3% minimum and an electrical conductivity of 53.5% IACS minimum.

characteristically this alloy is manufactured commercially by DC casting, reheating to about 700 to 850 F., hot rolling to redraw rod, cold drawing, solution izing at around 1000 F. for 1 hour or more followed by water quenching, additional cold drawing and artificial aging between 250 and 450 F. to specification properties.

Batch solutionizing is conventionally conducted on this alloy on relatively small, spaced coils of no more than 250 lbs. which must be carefully water quenched.

From a manufacturing point of view the solutionizing of drawn, intermediate diameter wire is an expensive and slow process. It is particularly diificult because of the quench sensitivity of this alloy. In fact, the entire foregoing operation adds substantially to the manufacturing cost of conductors of this type.

It is highly desirable to make this intermediate diameter resolutionizing operation in a continuous process. If this could be done, significant manufacturing economies would result. In addition, batch processing frequently results in hard spots in the resultant product and irreproducibility from length to length. Continuous processing would tend to eliminate these disadvantages.

Accordingly, it is principal object of the present invention to provide an improved process for the preparation of aluminum-magnesium-silicon alloy conductors.

It is a particular object of the present invention to provide a process as above which enables the attainment of good physical properties in the resultant article while achieving significantly reduced manufacturing cost.

Further objects and advantages of the present inention will appear hereinafter.

In accordance with the present invention it has now 3,418,177 Patented Dec. 24, 1968 been found that the foregoing objects and advantages may be readily achieved.

The present invention comprises:

(A)' Providing an aluminum base alloy containing from 0.20 to 1.3% silicon, from 0.3 to 1.4% magnesium, and the balance essentially aluminum;

(B) Holding said alloy at a temperature of from 950 to 1150 F. for at least 5 minutes;

(C) Hot rolling to redraw rod with a cooling rate during hot rolling of greater than 400 F. per minute;

(D) Cooling the redraw rod to room temperature;

(E) Holding at a temperature of 975 to 1100 F. for

greater than 15 seconds; and

(F) Cooling to room temperature at a rate of at least 400 F. per minute.

In the preferred embodiment the following additional steps are performed:

(G) Cold drawing with at least reduction in cross section area; and

(H) Heating at from 250 to 450 F. for at least 15 minutes.

The foregoing process significantly reduces the cost of the commercial preparation of aluminum base alloy conductors containing magnesium and silicon. The process described above is readily susceptible to use on a continuous commercial scale and effects significant manufacturing economies.

The process described above develops suitable structures in hot rolling so that flash solutionizing of magnesiumsilicide can be employed. Fash solutionizing on a continuous basis requires that the amount of magnesiumsilicide precipitate to be dissolved is relatively small in quantity and is finely dispersed in a uniform fashion. The time required for dissolution of the magnesiumsilicide is essentially the time required for magnesium to diffuse one-half the average interparticle distance. With normal direct chill casting and conventional hot rolling the magnesium-silicide precipitate is very coarse and widely distributed. Long periods of time are therefore required to redissolve this coarse precipitate. The coarse precipitate can be redissolved slowly by batch treatments and retained in solution by batch quenching. However, in batch quenching cooling rates and degrees of retention are frequently non-uniform and the resultant product may have hard spots with irreproducible characteristics from length to length.

In accordance with the present invention, it has been discovered that by the use of a carefully defined, nonconventional series of process steps, the distribution of magnesium-silicide in the hot rolled redraw rod is eminently suitable for rapid dissolution, thus resulting in a greatly improved process.

In accordance with the present invention the aluminum alloy may be conventionally cast in the customary manner. The cast bar is then completely solutionized by holding in the temperature range of 950 to 1150 F. for a period of time of at least 5 minutes and preferably at a temperature of 975 to 1050 F. for a period of time of between 15 minutes and 16 hours.

After the foregoing homogenization step, the solutionized cast bar is hot rolled into redraw rod, conventionally having a diameter of about /8 of an inch. The solutionized bar is hot rolled to redraw rod so that during hot rolling the cooling rate is greater than 400 F. per minute. By utilizing the foregoing hot rolling practice, the redraw rod exiting from the hot rolling mill will be substantially free of precipitated magnesium-silicide. During subsequent cooling of the warm redraw rod to room temperature the magnesium-silicide will precipitate in a fine and uniform- 3 1y dispersed fashion particularly suitable for rapid resolutionizing.

We have discovered that the flash solutionizing operation is best conducted in the redraw rod stage by heating, preferably rapidly, to within the temperature range of 975 to 1100 F. and holding at that temperature for no less than 15 seconds and preferably by heating rapidly to between 1000 to 1050 F. and holding at that temperature for between 30 to 120 seconds. This thermal cycle will serve to completely redissolve the specially precipitated magnesium-silicide and the strength of, for example, the 6201 alloy within this temperature range is always in excess of 1000 p.s.i. so that the redraw rod can be adequately handled mechanically.

Immediately after resolutionizing, the magnesium-silicide must be retained in solution by cooling to room temperature at a rate of no less than 400 F. per minute and preferably in excess of 1000 F. per minute. The delay time between solutionizing and quenching should be less than 3 minutes and preferably less than 30 seconds.

The solutionized redraw rod can then be cold drawn without an intermediate softening treatment in excess of 98% reduction in cross sectional area. This is in distinction to the present commercial process in which an upper limitation is applied to the amount of cold reduction after solutionizing. However, when aluminum alloy 6201 is utilized, it is necessary to apply at least 70% cold reduction, and preferably at least 75% cold reduction, before the final aging treatment.

Combinations of mechanical and electrical properties comparable or superior to those of present commercial material can be obtained at final diameter by the application of an appropriate and conventional softening treatment consisting of holding at between 250 and 450 F. for 15 minutes minimum. Preferably this treatment should be conducted between 300 to 350 F. for 1 to 8 hours. Specification and mechanical and electrical properties are obtained using the above process with cold reduction after solutionizing from 70 to 99% reduction in area without intermediate softening treatments.

The present invention is amenable to use with any aluminum base alloy containing from 0.20 to 1.3% silicon and from 0.3 to 1.4% magnesium, with the 6201 alloy being preferred. Naturally, conventional impurities may be present and additives may be employed to achieve particular results. For example, the alloy may contain 1.0% iron max., 1.0% copper max., 0.8% manganese max., 0.35% chromium max., 0.5% zinc max., boron 0.001 to 0.05%, 0.2% titanium max., and others each 0.05% max., total 0.15%. Boron is a particularly advantageous alloying addition.

The present invention will be more readily understandable from a consideration of the following illustrative examples.

Example I An alloy containing 0.72% magnesium, 0.68% silicon, 0.22% iron and 0.018% boron was horizontally DC cast to a diameter of 2%". The casting was reheated to around 750 F. held at that temperature for 30 minutes and rolled to redraw rod in a ten-stand, continuous hot mill to a diameter of The rod exited the rolling mill at 550 F. and was air cooled to room temperature. The resulting redraw rod had a coarse dispersion of Mg Si out of solution. The redraw rod was cold drawn 80% and aged for 4 hours at 325 F. Four samples of aged wire showed a low average tensile strength of around 35,000 p.s.i.

Example II The redraw rod of Example I was heated at 1025 F. for 10 minutes at temperature, total time in the furnace of 16 minutes, and then quenched in water without a deliberate intermediate delay. This served to solutionize the coarse Mg Si completely. The solutionized redraw rod was drawn 80% and aged at 325 F. for 4 hours. Eight tensile samples of the wire showed a satisfactory average tensile strength of 48,000 p.s.i. This essentially duplicates present commercial processing.

Example III The cast alloy bar of Example I was heated to 1025 F. and held at that temperature for 15 minutes-5 hours. It was then rolled in a ten-stand tandem mill to /8" redraw rod. The cast bar entered the rolling mill at 1025 F. and exited at 650 F. The average cooling rate during hot rolling was 1600 F./minute. The rod was then cooled to room temperature in still air at an average rate of around 10 F./ minute. Examination of the microstructure showed a fine dispersion of precipitated Mg Si. The redraw rod was cold drawn to 0.160" diameter and aged at 325 F. for 4 hours. Four samples of aged wire showed a low average tensile strength of around 35,000 p.s.i.

Example IV The redraw rod of Example III was heated for 30 seconds at 1025 F. followed by cooling to room temperature at an average cooling rate greater than 400 F. per minute. The redraw rod so treated showed no significant precipitation of Mg Si in its microstructure. It was then drawn 80% to 0.160" diameter and aged at 325 F. for 4 hours. Eight samples of the wire showed a satisfactory average tensile strength of around 48,000 p.s.i., an average elongation of around 5% and average electrical conductivity of around 54% IACS.

This invention may be embodied in other forms or carried out in other ways without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency are intended to be embraced therein.

What is claimed is:

1. The process for preparing wrought aluminum alloys which comprises:

(A) providing an aluminum base alloy containing from 0.20 to 1.3% silicon, from 0.3 to 1.4% magnesium and the balance essentially aluminum;

(B) holding said alloy at a temperature of from 950 to 1150 F. for at least 5 minutes;

(C) hot rolling to redraw rod with a cooling rate during hot rolling of greater than 400 F. per minute;

(D) cooling the redraw rod to room temperature;

(E) holding at a temperature of from 975 to 1100 F.

for greater than 15 seconds; and

(F) cooling to room temperature at a rate of at least 400 F. per minute.

2. The process of claim 1 including the following steps subsequent to step (F):

(G) cold drawing with .at least 70% reduction in cross sectional area; and

(H) heating at from 250 to 450 F. for at least 15 minutes.

3. A process for preparing aluminum alloy conductors according to claim 2 wherein said holding step (B) is at a temperature of 975 to 1050 F. for from 15 minutes to 16 hours.

4. A process according to claim 2 wherein said redraw rod (C) has a diameter of about Vs of an inch and is substantially free of precipitated magnesium-silicide.

5. A process according to claim 2 wherein said cooled redraw rod (D) contains fine, uniformly dispersed magnesium-silicide percipitate.

6. A process according to claim 2 wherein said holding step (E) is at a temperature of 1000 to 1050 F. for between 30 and seconds.

7. A process according to claim 2 wherein said cooling step (F) utilizes a cooling rate in excess of 1000 F. per minute.

8. A process according to claim 2 wherein the time 5 6 delay between said holding step (E) and cooling step (F) References Cited is less than 3 minutes. U T FATE 9. A process according to claim 2 wherein said heating NITED ES NTS step (H) is at from 300 to 350 F. for from 1 to 8 hours. 3,031,299 4/ 1962 Cnner 148115 10. A process according to claim 2 wherein said alloy 5 312191491 11/1965 Anderson at a! 148 11'5 is aluminum alloy 201 3,224,053 2/1966 PI'YOI 148-11- 11. A process according to claim 2 wherein said alloy 3'329537 7/1967 Loach 148 12-7 contains from 0.001 to 0.05% boron.

12. A process according to claim 2 wherein said alloy DEWAYNE RUTLEDGE, 'Y Examine"- contains 1.0% iron max., 1.0% copper max., 0.8% man- 10 STALLARD, Assistant Examiner ganese max., 0.35% chromium max., 0.5% zinc max., 0.2% titanium max., and others each 0.05 max, total U.S.Cl.X.R. 0.15%. 14812.7, 32.5 

